If co-op was included I wouldn't mind at all. I mean its not like you wouldn't be able to block a random from joining your game and fucking it up.
This is pie in the sky, but it would be pretty incredible to have three multiple characters stories unfolding at the same time if some of your friends were playing in the same world. Imagine you hop a ride on the subway, and as you are traveling to your destination you see one of your friends driving by on the streets below trying to outrun a wanted level. IMAGINE!
That's some next gen stuff my friends...
My more realistic guess is that they are simple offering the player three different perspectives of the overall story and world. Seems like you have your blue collar hill-billy who will probably live outside of the city. The gang member or thug, who will be more about gang wars and drug dealing. And then the middle aged man who lives a more posh and high class lifestyle. And you will just switch back and forth during the main story.
Remember the main theme of this game seems to be all about money. The three characters seem to represent the lower, middle, and high class of American society.
*edit* OH SHIT....DOWNLOADING NOW!!